1. Field of the Invention
The invention relates to raster scanned light modulation utilizing a coherent light source and producing a phase modulation of the light. More particularly, the invention relates to input optical transducers, such as a coherent light valve, in which surface deformations of the recording medium form a variable pitch grating for phase modulation of the light and in which phase error due to undesired thickness variation of the medium may occur and require correction. The invention also relates to a means for generating a variable frequency wave suitable for phase correction to the optical modulation system.
2. Description of the Prior Art
In raster scanned light modulators in which coherent light is phase modulated, the ultimate accuracy of the system can only be achieved if system tolerances are held to a fraction of a wavelength of light. In the event of thickness variation in the light modulating medium, the phase front may be retarded or advanced at different portions of the raster and in consequence, substantial error may be introduced into the system. When a light modulator is used in such applications as a spectrum analyzer in which a Fourier transformation occurs, terms which should be located at specified coordinates in the spatial frequency plane of the transforming lens may be blurred and of reduced output so that accurate interpretation is prevented.